The invention relates to a low-pressure discharge lamp provided with a discharge vessel which is closed in a gastight manner and which contains an ionizable filling, and further provided with electrodes for maintaining an electric discharge in the discharge vessel. At least one of the electrodes being positioned inside the discharge vessel, which electrode comprises a coil of a refractory metal which is electrically connected to current conductors which extend to outside the discharge vessel. The coil has a central portion which is coated with an electron-emitting material and has end portions on either side between the central portion and respective current conductors, which end portions are coated with a protective material.
The invention also relates to a compact fluorescent lamp.
Such a lamp is known from U.S. Pat. No. 5,233,268. The known lamp is a low-pressure mercury discharge lamp with a tubular discharge vessel in which an electrode of the kind described above is arranged at each end. The central portion of the coil of each electrode is coated with an electron-emitting material which is composed of a mixture of oxides of the alkaline earth metals barium, calcium, and strontium. Usually, the electron-emitting material in such lamps is provided in that the central portion is coated with a suspension of carbonates of said alkaline earth metals. After the electrodes have been positioned in the discharge vessel, but before the discharge vessel is closed, the electrodes are resistively heated in that a current is passed through them. This achieves a conversion of the carbonates into oxides, with carbon dioxide being released. The end portions of the electrodes, usually having a length of approximately 1 to a few millimetres, are usually kept free from electron-emitting material because these portions do not become hot enough during the heating process for a full conversion of the carbonates to take place in the suspension provided there. This would have the result that the conversion of carbonates takes place subsequently during lamp operation. The evolved carbon dioxide would seriously interfere with further lamp operation.
It is attractive to operate low-pressure discharge lamps by means of a supply unit with cold ignition, i.e. a supply unit which ignites the lamp without preheating the electrodes. Such a supply unit is comparatively simple and inexpensive because additional means for preheating the electrodes are unnecessary. The operational lives of lamps with such supply units, however, are mainly determined by their switching lives, i.e. the number of times the lamp can be switched on, in the case of short operational periods, i.e. shorter than one hour.
The current conductors to the electrode in the known lamp are each coated with a glass sleeve which also covers the end portions of the coil. This measure prevents the discharge arc from applying itself to end portions of the coil not coated with electron-emitting material in the case of cold ignition and thus prolongs the switching life of the lamp. It is a disadvantage, however, that the sleeves are comparatively difficult to fasten. This is particularly disadvantageous in the case of large-scale manufacture of the lamp.